A cryogenic storage tank is a cryogenic container for storing liquefied natural gas (LNG), liquid nitrogen, liquid oxygen, and other products, which generally comprises a housing, an inner container, and a vacuum insulation interlayer disposed between the housing and inner container. Additionally, there are straps disposed between the housing and inner container in order to maintain the strength and stability of the inner container.
Daily evaporation rate of a cryogenic tank is generally 0.02%˜0.08% when the tank is full. The better insulation is of the tank, the lower cooling loss, and the smaller evaporation rate; when the evaporation rate increases, the burden on the evaporation gas compressor and refrigeration system increases as well, which causes frequent re-starts, and as a result, increased energy consumption. The insulation effect of the vacuum insulation interlayer and the disposition of the straps are the main factors affecting the rate of evaporation.
The LNG stored in a cryogenic storage tank enters into the pump outside the tank body through a liquid outlet pipe provided on the tank body. Before LNG enters into the pump, if the pressure of saturation liquid decreases or the temperature increases, the liquid may be overly saturated so as to generate overflow bubbles, which will go with the flow of liquid from the low pressure side to the high pressure side of the pump. Under the high pressure, the bubbles are quickly condensed or ruptured, thereby causing pump cavitation that prevents the pump from normal operation, which will increase the operational cost and energy consumption. In other words, when more heat leaks into the area between the liquid outlet pipe and pump, the LNG temperature rises to the extent that LNG flashes before entering into the pump. As a result, the pump cavitation occurs, and so is cooling loss. If the gas (Boil off Gas, abbreviated BOG) that is formed as a result of gasification of LNG cannot be recycled effectively, it will further increase the waste of energy.
In addition, in order to achieve the desired saturation temperature during the use of LNG, there is a need to transport gas from an LNG vaporizer saturation adjustment mechanism into the LNG tank to facilitate rolling of the liquids in the tank. In existing technologies, the liquids in the LNG storage tank may flow backward into the pipeline so that the pipeline is filled with liquids over time, which results in an increase in BOG and subsequent diffusion, and further, significant waste of LNG.